Power semiconductor devices such as switching devices and diodes are used in the circuits of switching power supplies, inverters, and the like. Those power semiconductor devices are required to have high breakdown voltage and low on-state resistance. The relationship between breakdown voltage and on-state resistance is a trade-off relationship determined by device materials.
As technological development has progressed, the on-state resistance of power semiconductor devices has become close to the limit of silicon, which is a principal device material. So as to lower the on-state resistance further, a change of device materials is necessary. By using a nitride semiconductor like GaN or AlGaN, or a wide bandgap semiconductor like silicon carbide (SiC) as the switching device material, the trade-off relationship determined by materials can be improved, and the on-state resistance can be dramatically lowered.
An example of a device that uses a nitride semiconductor such as GaN or AlGaN and easily achieves low on-state resistance is a heterojunction field effect transistor (HFET) that uses an AlGaN/GaN heterostructure. This HFET realizes low on-state resistance with the high mobility of a heterointerface channel and the high concentration of electrons generated through polarization. Accordingly, low on-state resistance is achieved, even if the chip area of the device is small.
However, as the HFET generates electrons through polarization, electrons also exist at a high concentration under the gate electrode. Therefore, the HFET is a normally-on device that generally has a negative gate threshold voltage. For safe operation, a power semiconductor device is preferably a normally-off device that has a positive gate threshold voltage. So as to realize a normally-off device, the concentration of electrons under the gate electrode needs to be selectively limited to zero.